This patch addresses a couple of smaller issues with the `PDFHistory` class:
- Most, if not all, other viewer components can be reset in one way or another, and there's no good reason for the `PDFHistory` implementation to be different here.
- Currently it's (technically) possible to keep adding entries to the browser history, via the `PDFHistory` instance, even after the document has been closed. That obviously makes no sense, and is caused by the lack of a `reset` method.
- The internal `this._isPagesLoaded` property was never actually reset, which would lead to it being temporarily wrong when a new document was opened in the default viewer.
This major version bump required two changes:
- The global line in the mobile viewer example should be removed because
the `.eslintrc` file already defines these globals and with the new
`eslint` version we otherwise get an error saying "'pdfjsLib' is already
defined as a built-in global variable".
- The ECMA version for the examples must be set to 6 since we're using
modules, otherwise we get an error saying "sourceType 'module' is not
supported when ecmaVersion < 2015". It turns out that the previous
version of `eslint` already used ECMA version 6 silently even though
we set 5, see https://github.com/eslint/eslint/issues/9687#issuecomment-432413384,
so in terms of our code nothing really changes.
Rather than having two different (but connected) options for the textLayer, I think that it makes sense to try and unify this. For example: currently if `disableTextLayer === true`, then the value of `enhanceTextSelection` is simply ignored.
Since PDF.js version `2.0` already won't be backwards compatible in lots of ways, I don't think that we need to worry about migrating existing preferences here.
This removes the `PDFJS.useOnlyCssZoom` dependency from the viewer components, but please note that as a *temporary* solution the default viewer still uses it.
Since the `mobile-viewer` example is based on the old FirefoxOS/B2G PDF viewer, it didn't need to have the same kind of `open/close` methods as the default viewer.
However, now that it has been re-purposed as a simple `mobile-viewer` example, it seems like a good idea to ensure that it has proper asynchronous `open/close` methods.
Fixes 7571.
